MOS Technology, Inc. (pronounced [ɛm-oʊ-ɛs] or em-oh-ess) was a semiconductor design and fabrication company based in Norristown, Pennsylvanisa, USA. The term MOS is an acronym for Metal Oxide Semiconductor, a 3-layered electronic construction: a silicon wafer substrate underneath a grown layer of silicon dioxide (SiO2) underneath a layer of metal or polycrystalline silicon. MOS Technology, Inc. (referred to hereafter as MOS Technology) should not be confused with MOSTek.

MOS Technology was formed in 1969 to produce MOS integrated circuits by three executives from General Instrument: Mort Jaffe, Don McLaughlin, and John Pavinen. The company’s strapline was “We’ve turned a technology into a company”. In 1970, Allen-Bradley, a supplier of electronic components, acquired a majority interest. In 1970, MOS Technology began supplying Commodore with microchips for its calculator range.

On 19 August 1974, an electrical engineer named Chuck Peddle left Motorola and joined his old friend, John Pavinen, at MOS Technology. He was followed by seven other Motorola electrical engineers: Harry Bawcum, Ray Hirt, Terry Holdt, Mike James, Will Mathis, Bill Mensch and Rod Orgill. The team were frustrated with Motorola’s refusal to provide integrated circuit accessibility for a general consumer market through the design and production of a low cost microprocessor. On 30 October 1974, realising that a significant intellectual potential had gone, Motorola filed several patent applications relating to microprocessors.

By June 1975 MOS Technology had created a few working versions from their new MOS Technology 650x family of microprocessors; with prices set at US$20-$25 compared to US$200 for the Motorola 6800. The MOS Technology 6501 was pin compatible with the Motorola 6800 and, although not software compatible, threatened to undermine Motorola’s position. The MOS Technology 6502 was a slight variation on the MOS Technology 6501, with extra I/O pins, and was not pin compatible with the Motorola 6800. The MOS Technology 6502 would soon be installed as the CPU in Apple, Atari, Commodore, and NES microcomputers and video game consoles.

In 1975 Texas Instruments (TI) entered the calculator business, selling complete products for less than the cost of microchips bought by existing calculator companies. In the ensuing ‘Calculator War’, prices dropped and many companies went out of business. Those that survived found other industries for their microchips. MOS Technology survived by becoming a supplier to Atari and produced the custom single-chip for the Pong video game system.

Although interest was high at the WESCon (Western Electronic Show and Convention) in September 1975, and a spooked Motorola reduced the cost of the 6800 from US$175 to US$69, sales of the 6501 were suspended almost immediately. On 3 November 1975 Motorola sought an injunction in Federal Court to stop MOS Technology from making and selling microprocessor products. They also filed a lawsuit citing patent infringement and misappropriation of trade secrets. Motorola claimed that MOS Technology was essentially making their component, and that Mike James took confidential documents pertaining to the 6800 when he left. Although it is possible that MOS Technology would have won, Motorola was a billion-dollar company with a strong case.

Nevertheless, Allen-Bradley decided not to fight this case and sold their interest in MOS Technology back to the founders. In March 1976, the now independent MOS Technology was low on funding and decided to settle the case. They would cease production of the 6501, pay Motorola $200,000 in damages and return the confidential documents. Both companies agreed to cross-license microprocessor patents. Notwithstanding the outcome, the 6502 would continue to be produced and in May 1976 Motorola responded by dropping the 6800 price to US$35.

On 6 July 1976, a patent was granted to Motorola (crediting William H. Mensch, Jr. as the inventor) entitled Chip Topography for MOS Interface Circuit[1]. This patent covered the architecture of a MOS peripheral interface adaptor chip and the connectivity to other components.

In April 1976 MOS Technology built the KIM-1 single-board computer as their 650x products could not use the architecture of existing PCBAs. It featured a single 6502 microprocessor, two 6530 arrays, machine language monitor on 2 kB ROM, 1 kB RAM, fifteen I/O pins, a hexadecimal keypad, LED display, an audio cassette interface and a serial teleprinter interface. It cost US$245.

By 1976, Commodore was the largest purchaser of integrated circuits from MOS Technology. As TI had integrated vertically upward to create calculators from their chip manufacturing business, so Jack Tramiel decided that Commodore would compete by integrating vertically downward into the integrated circuit business. MOS Technology was struggling financially and, with additional backing from Irving Gould, Commodore acquired MOS Technology. On 9th September 1976 the New Scientist journal reported[2]:

"Commodore takes over chip manufacturer MOS Technology, Inc. of Pennsylvania, after being closely associated for some years... Commodore ...has acquired 100 per cent of the equity of MOS Technology, Inc. of Pennsylvania in exchange for a 9.4 per cent equity stake in Commodore. MOS Technology is privately owned and valued at around $12 million.”

MOS Technology continued to manufacture integrated circuits for other companies, but essentially enabled Commodore to reduce massive costs and ensure supply. The name remained so that existing literature did not have to be reproduced.

By 1983 MOS Technology become the Commodore Semiconductor Group (CSG), and references to CSG are made in the Commodore 64 Programmer's Reference Guide[3]. Despite this, the MOS Technology logo continued to be printed on CSG integrated circuit casings until 1989.

After the ruin of Commodore in 1994 the CSG was sold to the former MOS management and the production of MOS chips was restarted in 1995 in Norristown, Montgomery County, Pennsylvania.
In this time the company operated under the name Great Mixed-signal Technologies (short: GMT Microelectronics). After contaminating the local groundwater with trichloroethylene and other volatile organic compounds through a tank leak the company closed in 2001.